Manufacturing of semiconductor devices, in particular integrated circuits having multiple-layered structures with various metal and non-metal layers laminated on a semiconductor substrate, typically involves an application of several metal layers onto a substrate or onto other previously deposited layers. These layers may have a complicated planar topology since these layers may constitute thousands of individual devices, which in combination form an integrated circuit or so-called “chip”. Modern chips may have metal or dielectric layers with thicknesses from tens of Angstroms to fractions of a micron.
It is understood that thin metallic films used in integrated circuits of semiconductor devices function as conductors of electric current. Furthermore, it is known that densities of signal currents in metallic interconnections used in integrated circuits may reach extremely high values that generate such phenomena as electromigration associated with spatial transfer of mass of conductor films. Therefore the characteristics and properties of the deposited metal films (uniformity of film thickness, low electrical resistivity, etc.) determine performance characteristics and quality of the integrated circuit and of the semiconductor device as a whole.
In view of the above, thin metal films used in integrated circuits should satisfy very strict technical requirements relating to metal deposition processes, as well as to repeatability and controllability of the aforementioned processes.
A wide range of metals is utilized in the microelectronic manufacturing industry far the formation of integrated circuits. These metals include, for example, nickel, tungsten, platinum, copper, cobalt, as well as alloys of electrically conductive compounds such as silicides, solders, etc. It is also known that coating films are applied onto substrates with the use of a variety of technological processes such chemical vapor deposition (CVD), physical vapor deposition (PVD), electroplating, and electroless plating. Of these techniques, electroplating and electroless plating or deposition tend to be the most economical and most promising for improvement in characteristics of the deposited films. Therefore, electroplating and electroless plating techniques successfully replace other technologies.
Electroplating and electroless plating can be used for the deposition of continuous metal layers as well as patterned metal layers. One of the process sequences used by the microelectronic manufacturing industry to deposit metals onto semiconductor wafers is known to as “damascene” processing. In such processing, holes, commonly called “vias”, trenches and/or other recesses are formed on a workpiece and filled with a metal, such as copper. In the damascene process, the wafer, with vias and trenches etched in the dielectric material, is first provided with a metallic seed layer, which is used to conduct electrical current during a subsequent metal electroplating step. If a metal such as copper is used, the seed layer is disposed over a barrier layer material, such as Ti, TiN, etc. The seed layer is a very thin layer of metal, which can be applied using one or more processes. For example, the seed layer of metal can be laid down using physical vapor deposition or chemical vapor deposition processes to produce a layer with the thickness on the order of 1,000 Angstroms. The seed layer can advantageously be formed of copper, gold, nickel, palladium, or other metals. The seed layer is formed over a surface, which may contain vias, trenches, or other recessed device features.
A metal layer is then electroplated onto the seed layer in the form of a continuous layer. The continuous layer is plated to form an overlying layer, with the goal of providing a metal layer that fills the trenches and vias and extends a certain amount above these features. Such a continuous layer will typically have a thickness on the order of 5,000 to 15,000 Angstroms (0.5-1.5 microns).
After the continuous layer has been electroplated onto the semiconductor wafer, excess metal material present outside of the vias, trenches, or other recesses is removed. The metal is removed to provide a resulting pattern of metal layer in the semiconductor integrated circuit being formed. The excess plated material can be removed, for example, using chemical mechanical planarization. Chemical mechanical planarization is a processing step, which uses the combined action of chemical removal agents, or a chemical removal agents with an abrasive, which grinds and polishes the exposed metal surface to remove undesired parts of the metal layer applied in the electroplating step.
Disadvantages associated with electroplating are technical problems in connection with designing of reactors used in the electroplating of semiconductor wafers. Utilization of a limited number of discrete electrical contacts (e.g., 8 contacts) with the seed layer about the perimeter of the wafer ordinarily produces higher current densities near the contact points than at other portions of the wafer. This non-uniform distribution of current across the wafer, in turn, causes non-uniform deposition of the plated metallic material. Current thieving, affected by the provision of electrically conductive elements other than those, which contact the seed layer, can be employed near the wafer contacts to minimize such non-uniformity. But such thieving techniques add to the complexity of electroplating equipment, and increase maintenance requirements.
The specific metal to be electroplated can also complicate the electroplating process. For example, electroplating of certain metals typically requires use of a seed layer having a relatively high electrical resistance. As a consequence, use of the typical plurality of electrical wafer contacts (for example, eight discrete contacts) may not provide adequate uniformity of the plated metal layer on the wafer. Reduction in sizes of such features as vias and trenches also requires thinner layers having higher resistivity, which in turn may generate a high potential drop from the wafer edges to the central part, whereby the rate of deposition in the central area is significantly reduced.
Beyond the problems discussed above, there are also other problems associated with electroplating reactors. As device sizes decrease, the need for tighter control over the processing environment increases. This includes control over the contaminants that affect the electroplating process. The moving components of the reactor, which tend to generate such contaminants, should therefore be subject to strict isolation requirements.
Still further, existing electroplating reactors are often difficult to maintain and/or reconfigure for different electroplating processes. Such difficulties must be overcome if an electroplating reactor design is to be accepted for large-scale manufacturing.
One drawback associated with copper deposition by electroplating is the fact that for very small features on microelectronic workpieces (sub 0.1 micron features), copper deposition by electroplating can lack conformity with the side walls of high aspect ratio vias and trenches, and can produce voids in the formed interconnects and plugs (vias). This is often due to the non-conformity of the copper seed layer deposited by PVD or CVD. As a result, the seed layer may not be thick enough to carry the current to the bottom of high aspect ratio features.
An alternate process for depositing copper onto a microelectronic workpiece is known as “electroless” plating which is the deposition of metals on a catalytic surface from a solution without an external source of current. For example, this process can be used as a preliminary step in preparing plastic articles for conventional electroplating. After cleaning and etching, the plastic surface is immersed in solutions that react to precipitate a catalytic metal in situ, palladium, for example. First the plastic is placed in an acidic stannous chloride solution, then into a solution of palladium chloride; palladium is reduced to its catalytic metallic state by the tin. Another way of producing a catalytic surface is to immerse the plastic article in a colloidal solution of palladium followed by immersion in an accelerator solution. The plastic article thus treated can now be plated with nickel or copper by the electroless method, which forms a conductive surface, which then can be plated with other metals by a conventional electroplating method.
Along with the electroplating method, the electroless method also has found wide application in the manufacture of semiconductor devices. Since the problem of heating and cooling of a workpiece, such as a semiconductor substrate is especially pertinent for electroless deposition processes, the following description will be continued with specific reference to electroless deposition. It is understood, however, that the electroless process is given only as a example and that the term “temperature controlled substrate holder” is equally applicable to processing in any process that involves treatment of a temperature controlled object in a fluid.
As compared to electroplating, electroless plating or deposition is a selective process, which can be realized with very thin seeds or without the use of seeds at all. Since an electroless process is not associated with the use of an external electric current source, electroless deposition results in more uniform coatings in view of the absence of discrete contacts. Electroless deposition can be realized with the use of simple and inexpensive equipment and with a high aspect ratio gap fill.
Given below are several examples of existing methods and apparatuses for electroless deposition, specifically for use in the manufacture of semiconductor devices.
For example, U.S. Pat. No. 5,500,315 issued in 1996 to J. Calvert, et al. discloses an electroless metal plating-catalyst system that overcomes many of the limitations of prior systems. In one aspect of the invention, the process comprises the steps of: providing a substrate with one or more chemical groups capable of ligating to an electroless deposition catalyst, at least a portion of the chemical groups being chemically bonded to the substrate; contacting the substrate with the electroless metal plating catalyst; and contacting the substrate with an electroless metal plating solution to form a metal deposit on the substrate. The chemical groups can be, for example, covalently bonded to the substrate. In another preferred aspect, the invention provides a process for selective electroless metallization, comprising steps of selectively modifying the reactivity of a substrate to an electroless metallization catalyst; contacting the substrate with the electroless metallization catalyst; and contacting the substrate with an electroless metallization solution to form a selective electroless deposit on the substrate. The substrate reactivity can be modified by selective treatment of catalyst ligating groups or precursors thereof on the substrate, for example by isomerization, photocleavage or other transformation of the ligating or precursor groups. Such-direct modification enables selective plating in a much more direct and convenient manner than prior selective plating techniques. Specifically, the aforementioned patent provides selective electroless deposition without the use of a photoresist or an adsorption type tin-containing plating catalyst.
Although the above method provides selective patterning of substrate ligating groups and although some of practical examples indicate different temperatures of the solution, the invention does not teach methods for optimization of the electroless deposition process by controlling temperature of the solution or substrate holder. Furthermore, the aforementioned invention relates exclusively to a method and does not describe any electroless deposition equipment.
U.S. Pat. No. 6,309,524 granted to D. Woodruff, et al. in 2001 discloses, in one of its embodiments, a universal electroplating/electroless reactor for plating a metal onto surfaces of workpieces. An integrated tool for plating a workpiece comprises a first processing chamber for plating the workpiece using an electroless deposition process and a second processing chamber for plating the workpiece using an electroplating process. A robotic transfer mechanism is used that is programmed to transfer a workpiece to the first processing chamber for electroless deposition thereof and, in a subsequent operation, to transfer the workpiece to the second processing chamber for electroplating thereof.
It should be noted that a common problem in using bathes, which is especially true for the electroless deposition process, is that foreign particles or contaminants will be deposited on the substrate surface of the wafer when transferring the wafers from one bath to another bath. Another common problem is the exposure of the substrate surface of the wafer to air during the transfer (from bath to bath) can cause the non-wetting of deep and narrow trenches in the surface or small (contact) holes in the surface because of electrolyte evaporation. And yet another common problem is that exposure to air may cause oxidation of the catalytic surface that will result in poor catalytic activity and poor quality metal deposits. This problem becomes especially troublesome when using materials such as copper that easily oxidize in air. To produce high quality metal deposits in the submicron range, therefore, it is more desirable not to transfer the wafer between the processing chambers and to avoid exposing the wafer to air by using a single bath or processing chamber and moving the different fluids for each step in the process through the processing chamber. Furthermore, this patent also ignores such an important issue as temperature control of the solution and substrate.
The above problems are solved by the system described in U.S. Pat. No. 5,830,805 issued in 1998 to Y. Shacham-Diamand, et al. This patent discloses an electroless deposition apparatus and method of performing electroless deposition for processing a semiconductor wafer that use a closed processing chamber to subject the wafer to more than one processing fluid while retaining the wafer within the chamber. The invention is useful for manufacturing processes that include depositing, etching, cleaning, rinsing, and/or drying. The processing chamber used in the preferred embodiment of the apparatus of the above patent is an enclosed container capable of holding one or more semiconductor wafers. A distribution system introduces a first fluid into the chamber for processing the wafer and then removes the first fluid from the chamber after processing the wafer. The distribution system then introduces the next fluid into the chamber for processing the wafer and then removes the next fluid from the chamber after processing the wafer. This procedure continues until the manufacturing process finishes. The fluids used in the present invention depends on the process performed and may include fluids such as DI water, N2 for flushing, and electrolytic solutions comprising reducing agents, complexing agents, or pH adjusters.
The fluid enters the sealed processing chamber through an inlet, and exits the chamber through an outlet. As the fluid enters the processing chamber, the fluid is dispersed across the wafer in a uniform flow. A recirculation system moves the fluid through the processing chamber using a temperature control system, chemical concentration monitoring system, pump system, and a filtration system before re-circulating the fluid back through the processing chamber.
Additional embodiments include: a rotatingly mounted tubular wafer housing with a wafer mounted on either or both sides of the housing surface; an inner core mounted inside of the tubular housing when mounting a wafer on the inside surface of the housing; and a dispersal apparatus for dispersing the fluid in a uniform flow over the wafer. The processing chamber can be provided with a heater and a temperature control system. However, the processing chamber is an open-type chamber, which cannot be sealed and therefore does not allow pressure-controlled deposition processes. In addition, the open-type chamber does not provide adequate protection of the process against contamination of the solution.
In spite of their advantages, the known electroless processes have temperature of the working chemical solution as one of the main parameters. It is known that speed of deposition in an electroless process depends on the temperature in a degree close to exponential. For example, an article entitled “Electroless Nickel Plating,.” Finishing Publications Ltd., 1991, W. Riedel states (page 39 of the article) that temperature is the most important of parameters affecting the deposition rate and that for a Ni—P electroless process the deposition rate increases twofold for every 10 degrees of bath temperature.
Furthermore, for the metal interconnects on the surface of the wafer one of the major requirements is low resistivity. Copper was chosen as the close second best for fulfilling this requirement however, due to the presence of various additives in the interface between the PVD Cu seeds and ECD (electroplating copper deposition) Cu, resistivity is disproportionally increased as compared to much thinner electroless-deposited Cu layers. This phenomenon was reported by S. Lopatin at AMC, 2001.
It has been also shown by Y. Lantasov, et al. in “Microelectronics Engineering”, No. 50 (2000), pp. 441-447, FIG. 2, that resistivity of ELD Cu strongly depends on deposition conditions, and that at higher temperatures it is possible to obtain a material with low resistivity.
However, it is understood that electroless deposition at high temperatures leads to significant non-uniformities in the deposited layers. This occurs due to local temperature fluctuations. The higher is the temperature, the greater are such fluctuations. Stabilizalion of elevated temperatures in large volumes of the solution tanks is associated with the use of complicated temperature control systems and temperature maintaining systems (seals, thermal insulations, This, in turn, increases the cost of the equipment and maintenance.
For the reasons described above, manufacturers of semiconductor equipment prefer to use electroless processes carried out at room temperature. Low speeds of deposition are compensated by utilizing a multiple-station deposition equipment with simultaneous operation of a number of substrates in a number of chambers arranged in series (see, e.g., U.S. Pat. No. 6,322,677 issued in 2001 to D E. Woodruff, et al.). Such equipment requires a large production space and dictates the use of large volumes of the solutions. Furthermore, an additional space is needed for the preparation, storage, and post-use treatment of the solutions. This, in turn, creates environmental problems.
Another common drawback of existing electroless deposition apparatuses is low speed of deposition, which in general depends on the type of the deposited material and even in the best case does not exceed 100 nm/min, but normally is much lower. For example, for COWP the speed of deposition can be within the range from 5 nm/min to 10 nm/min.
In earlier U.S. patent application Ser. No. 10/103,015 filed on Mar. 22, 2002, the applicants have substantially solved the problems of eletroplating and electroless deposition associated with processes and apparatuses described above. More specifically, the apparatus described in the aforementioned patent application has a closable chamber that can be sealed and is capable of withstanding an increased pressure and high temperature. The chamber contains a substrate holder that can be rotated around a vertical axis, and an edge-grip mechanism inside the substrate holder. The deposition chamber has several inlet ports for the supply of various process liquids, such as deposition solutions, DI water for rinsing, etc., and a port for the supply of a gas under pressure. The apparatus is also provided with reservoirs and tanks for processing liquids and gases, as well as with a solution heater and a control system for controlling temperature and pressure in the chamber. The heater can be located outside the working chamber or built into the substrate holder, or both heaters can be used simultaneously. Uniform deposition is achieved by carrying out the deposition process under pressure and under temperature slightly below the boiling point of the solution. The solution can be supplied from above via a showerhead formed in the cover, or through the bottom of the chamber. Rinsing or other auxiliary solutions are supplied via a radially moveable chemical dispensing arm that can be arranged above the substrate parallel thereto.
The apparatus of U.S. patent application Ser. No. 10/103,015 provides uniform heating of the entire working solution by means of a heater located either outside of the deposition chamber with heating of the solution on the way to the chamber, or inside the cover of the deposition chamber. The main idea is to maintain the entire volume of the working solution at a uniform temperature. In general, the temperature can be adjusted, but should remain constant and at a relatively high level (e.g., 80 to 90° C.) all the time. However, although an elevated temperature of the working solution leads to essential increase in the productivity of the deposition process, the process requires constant replacement of the working solution since high temperature causes rapid thermal decomposition of the solution. Constant replacement of the solution should be carried out with high flow rates, and this, in turn, increases the cost of the production.
The undesired effect of permanent high temperature on the working solution can be explained as follows: Electroless deposition is a process of reduction of metal ions, e.g., cobalt, tungsten, or the like, on the catalytically-active surface by electrons released during oxidation of a reducing agent (e.g., hypophosphite anions). Oxidation of the reducing agent is catalyzed by a substrate, and, in the case of the most widely accepted model, it can be assumed that the charge from the reducing agent is transferred to metal ions through the substrate and thus produces metal atoms on the substrate surface.
A simplified combined chemical reaction for the above process can be expressed as follows:H2PO2−+H2O+Co++=Co°+H++H2PO3
The reducing agent is consumed by deposition of metal on the processed part, by hydrolysis at high temperature (especially on hot spots of heating elements), by catalytic oxidation on particles generated by the deposition tool hardware, and by a reaction of the reducing agent with reactive components (such as ethylene bonds, carboxyl groups. etc.) of the polymers used in tool construction.
At a sufficiently high concentration of contamination particles and/or at the boiling point of the solution, the composition can be spontaneously and completely decomposed by metal reduction at the surfaces of the aforementioned particles (and defects). As soon as few metal atoms are formed, they become new nucleation sites for further continuous decomposition of the solution.
International Patent Application Publication No. WO 02/34962 (hereinafter referred to as International Application) disclosed on May 2, 2002 describes an electroless apparatus, in which the problem of thermal decomposition of the working solution is partially solved by utilizing a substrate holder with a heating device. The substrate holder of this apparatus has a substrate chuck for clamping the substrate during deposition in the working chamber in a position of the treated surface facing down.
The main disadvantage of the apparatus of the aforementioned International Application is that the substrate is oriented with the treated surface facing down. It is known that in a static condition of the solution or in processes with low-velocity flows of the solution, the aforementioned orientation of the substrate leads to accumulation of gas bubbles on the treated surface. The gas bubbles, in turn, violate conditions required for uniformity of deposition. In order to solve this problem, in the apparatus of the International Application the deposition chamber has a curvilinear bottom surface for guiding the flows of the working solution in specific paths over the facing-down surface of the substrate. However, even though the flows of the working solution generate some dynamic conditions on the edge surfaces of the substrate, a certain stagnation point will always remain in the central part of the substrate. This local area may accumulate gas bubbles. Furthermore, differential of velocities of the flow on the substrate surface may lead to non-uniform temperature distribution. In other words, the apparatus of the International Application does not provide uniformity of electroless deposition.
In order to eliminate problems associated with accumulation of gas bubbles and stagnation of the solution in the central area of the chuck, one of the embodiments of the aforementioned apparatus includes a complicated kinematic system with wobbling motions of the rotating chuck. Such a complicated system makes the apparatus and products more expensive, while the process becomes difficult to control.
The disadvantages of the aforementioned International Application are eliminated by the substrate chuck disclosed in the U.S. patent application Ser. No. 10/24,331 filed by the same applicant and now pending. This application describes an apparatus based on the use of a substrate holder with a heater and cooler built into the body of the holder for selective cooling or heating of the surface of the substrate supported by the holder. According to one embodiment, the substrate holder has an electric-resistance type heater built into the chuck body. The supply of electric current to the heater is carried out through sliding current contacts which are connected to the heater by conductors and which are in contact with current collecting rings on the rotating shaft of the chuck. The chuck also contains a cooling unit built into its body below the heater and intended for rapid cooling of the chuck, when it may be required by a technological process. The cooling unit can be made in the form of a spiral channel for the passage of a cooling medium such as deionized water or the like. For this purpose, the cooling unit may be connected to a reservoir with a coolant. Circulation of the coolant is ensured by a provision of a pump installed in a line between the reservoir and the cooling unit. A manifold for collection and distribution of the coolant between the reservoir and the cooling unit consists of a stationary part and a rotating part attached to the shaft with respective sealing devices therebetween.
Although the device of this embodiment can be efficiently used for quick cooling of the heater, the heat is transferred to the substrate indirectly, i.e., through the material of the chuck. Another disadvantage is a complicated construction that requires electrical wiring inside the chuck body together with the formation of channels for the passage of the cooling fluid.
In the second embodiment of the temperature-controlled chuck for electroless deposition disclosed in U.S. patent application Ser. No. 10/242,331, a cooling unit and a heating unit are combined into a single Peltier-type cooler-heater, which is built into the body of the chuck and comprises a package of two semiconductor plates. These plates operate on the principle of generation of heat, when the current flow in one direction, and of absorption of heat, when the current flows in the opposite direction. Direction of the current is changed through the use of a switch that changes polarity on the semiconductor plate. A provision of the Peltier-type heater-cooler makes it possible not only to heat the working surface of the chuck, and hence of the substrate W, but also to quickly cool the holder surface and the substrate W.
In this device, similar to the previous one, heating and cooling are carried out indirectly via the material of the chuck body and therefore the construction of the cooler/heater that incorporates a Peltier-type unit possesses some cooling and heating inertia.
U.S. Pat. No. 5,775,416 issued in 1998 to D. Heimanson, et al., describes a temperature controlled chuck for vacuum processing that includes a heating unit and a cooling unit, both with the use of a gas. A first cavity separates the heating unit from a wafer substrate, and a second cavity separates the cooling unit from the heating unit. A gas delivery system conducts gas to the first cavity to facilitate exchange of heat between the heating unit and the substrate. A second gas delivery system conducts fluid to the second cavity to facilitate exchanges of heat between the heating unit and the cooling unit. A control system raises the temperature of the substrate by increasing power to the heating unit and by evacuating gas from the second cavity and lowers the temperature of the substrate by reducing power to the heating unit and by conducting gas to the second cavity.
An advantage of such a cooling/heating system is that the gas is maintained in a direct heat-transfer relationship with the wafer as it is supplied to a heating/cooling chamber, one wall of which is defined by the bottom of the wafer. However, this device utilizes gas as a cooling and heating medium inside the substrate holder itself. Increase of temperature is carried out through a built-in electrical heater, and the fluid is used as an auxiliary medium through which the heat is transferred. The system has thermal inertia and requires the use of complicated gas distribution and sealing units. As the pressure inside the aforementioned heating/cooling chamber and the pressure in the vacuum processing chamber above the wafer are not in balance, the wafer should be positively fixed inside the substrate holder. This is done by means of a ring clamp. Without the use of such a clamp, the wafer would have been raised from its seat in the substrate holder.
More specifically, efficient transfer of heat requires that the gas inside the substrate holder, i.e., under the wafer W, be maintained under pressure substantially higher than the reduced pressure in the working chamber above the water W. This pressure difference deforms the membrane-like wafer with the convex part facing upward. In order to prevent this phenomenon, a gas under pressure should be supplied to the working chamber for pressure balance. This is not always allowed by the technological process and, even if possible, is time-consuming and requires the use of complicated process-sequence control.